The present invention relates to television composite video decoders, and more particularly to an improved television composite video decoder for recovering luminance and chrominance information from an encoded color television signal by decoding each pixel in the same manner using a weighting technique which includes the pixels surrounding the pixel being decoded.
A video encoder, such as an NTSC encoder, typically receives signals representing the instantaneous magnitudes of red, green and blue light received by a television camera, or other component video device, as an image is scanned. These three signals are combined to produce a luminance signal referred to as "Y". The luminance signal describes the brightness of the image. Also produced are two signals that are used to describe the color characteristics of the image, the chrominance signals. In the NTSC encoding scheme these chrominance signals are referred to as "I" and "Q", and in the PAL format the somewhat different chrominance signals are referred to as "U" and "V". Regardless of the encoding scheme, or format, the chrominance signals will be referred to generically as Chroma 1 (C1) and Chroma 2 (C2). The C1 and C2 signals are limited in frequency to a maximum of 1.5 MHz. A reference sine wave signal, called the subcarrier, also is received by the video encoder. From the subcarrier signal the video encoder creates two sine waves of the subcarrier frequency that are in quadrature, i.e., having a phase difference of ninety degrees between the two sine waves. As shown in FIG. 1 the C1 signal 10 modulates the amplitude of one sine wave 11 and the C2 signal 12 modulates the amplitude of the other sine wave 13. The two modulated sine waves, C1 CHROMA and C2 CHROMA, are summed to create the chrominance signal, CHROMA SUM. This process of encoding the chrominance signal commonly is known as "quadrature amplitude modulation." Finally the chrominance signal is summed with the luminance (LUMA) signal Y to form a COMPOSITE video signal.
There are four very significant points in each cycle of each sine wave. The first two points are the two zero crossings 14. Regardless of the amplitude of the controlling chroma signal, the modulated signal is zero at these points in time. The other two significant points are the peaks 16 of the sine waves. The magnitude of the positive and negative peaks with respect to zero is the magnitude of the controlling chroma signal. Positive peaks indicate the C1 or C2 values, while negative peaks indicate the -C1 or -C2 values. Since C1 and C2 can be positive or negative themselves, a negative chroma signal value inverts the peaks. Further since the two original sine waves are in quadrature, when one is at a peak the other is at zero. By sampling the encoded chrominance signal at four times the subcarrier frequency at the proper phase, every four samples produce +C2, +C1, -C2, -C1. The C2 samples fall between the C1 samples and each alternate between positive and negative samples. Since the encoded composite video signal includes the luminance signal Y, sampling at four times subcarrier frequency results in samples, or pixels, of Y+C2, Y+C1, Y-C2, Y-C1. The problem for a video decoder is to take these sums and determine how much of each sum is due to the chrominance signal and how much is due to the luminance signal.
In the overall picture as shown in FIGS. 3A and 3B, which represents a small section 18 of a television screen 19, a video frame 20 has two interlaced video fields 21, 23 with the phase of the subcarrier frequency reversing from line to line within each field. Successive video frames 20 also have the phase of the subcarrier frequency reversed so that the corresponding lines of the same video field, 21 or 23, from frame to frame have the opposite subcarrier phase. The black lines 22 in the video fields 21, 23 of FIG. 3B indicate the lines which are provided by the opposite video field to make up the video frame 20. Looking at the pixels within one field, a pixel which contains a Y+C1 sum will have pixels above and below in the same field with Y-C1 sums. The corresponding pixel in the next and previous frames will contain a Y-C1 sum. Further looking only at the C1 pixels, the pixels in the same line on either side of the pixel containing the Y+C1 sum contain Y-C1 sums.
With this basic understanding of the operation of a video encoder many schemes have been used over the years to recover luminance and chrominance information from the composite video signal. A common scheme still employed in many color television receivers is to use a band pass filter centered at the subcarrier frequency to recover the chrominance information. The filter typically has a bandwidth of approximately 1.0 MHz. The luminance information is recovered by using a notch filter to delete the chrominance band, leaving high and low frequency luminance. The problems with this and similar approaches are that (a) luminance information near the subcarrier frequency is lost, (b) some luminance information near the subcarrier frequency is misinterpreted as chrominance, and (c) high frequency chrominance components bypass the notch filter and appear as luminance, producing an annoying "crawl" at extreme color transitions within the picture.
A more sophisticated approach involves what is known as comb filtering. In its simplest form a delay line with a delay equal to one horizontal line is used. This makes it possible to have pixels from two consecutive lines within a field at the same horizontal position available simultaneously. If a given pixel is currently available, the delay line provides a corresponding pixel from the previous line in the same column. If the prior pixel is a Y+C1 sum, the current pixel is a Y-C1 sum. Adding the two pixels together cancels the C1 term, leaving 2Y from which the luminance component Y is obtained by dividing by two. Likewise subtracting the two pixels from each other and dividing by two results in obtaining C1. The problem with this technique is that an assumption is made that the Y and C values have not changed very much from one line to the next. The greater the change, the greater the error In order to improve the results a narrow band subcarrier notch filter is placed in the luminance channel. The chrominance information is filtered to remove high frequency components. These high frequency components may be restored to the luminance channel in some designs. An improved version of this method uses two delay lines, combining the pixels from the prior and current lines, combining the pixels from the current line and the next line, combining the two resultants and dividing by four. This tends to reduce errors, but does not eliminate them. There are many variations of the above schemes, but all produce significant errors.
Another major decoding scheme is the adaptive comb filter. Basically decisions are made as to whether the vertical comb filtering described above should be enabled or not. If it appears that there is a great deal of change from line to line, vertical combing is disabled. Since the subcarrier phase reverses from frame to frame, if a given pixel is Y+C1 in a prior frame, it is Y-C1 in the current frame. Again if it appears that there is a great deal of change from frame to frame, this combing is disabled. This frame combing requires up to two frames of delay if combing is to be performed both forward and backward in time. It is apparent that combing may be performed in any direction: up, down, left, right, forward or backward. However this approach has two flaws. The first flaw relates to picture content. If there is motion in the scene, or picture image, each frame is different and frame combing produces errors. Within a field transitions may be vertical, horizontal or a combination thereof. In fact it is possible to have a single pixel that does not match any surrounding pixel in terms of Y and/or C values. The second flaw lies in the decision making process. Since the direction in which to comb is determined by the Y and C values which are unknown until decoded, the "solution" is to comb in all directions, vertically, horizontally and frame to frame, and look for a match between any two of them. However thin diagonal lines in the picture can trick the decoder into combing incorrectly. The result can be a significant loss of detail. Further usually none of the three comb values will be the same. Averaging of the closest pair cf comb values still provides the wrong answer, but hopefully not too wrong. Again there is a loss of fine detail in the picture. And when there is motion, frame to frame decoding must be disabled, forcing the video decoder to rely only on the current field information.
However, until now the adaptive comb filters have produced the best results by attempting to extract luminance and chrominance values using those pixels judged to be closest in value to the pixel being decoded. Since the decision making process is flawed, and because the surrounding pixels are not always close enough in value to provide an accurate decode, the resulting picture exhibits a visible loss of fine detail as well as crawl at the edges of brightly colored objects.
What is desired is a video decoder to more accurately decode luminance and chrominance information from an encoded television signal without the requirement of a decision making process and regardless of the motion within a picture.